I've known about the TrigpointingUK app for a while but only got around to installing it the other day.
Very useful for viewing 1:50000 and 1:25000 scale OS maps. The mapping seems to download very quickly, and the best bit is that it caches it - handy when you're out and about with little or no data connectivity.
It can also display a few other map formats, e.g. Open Cycle Map, Map Quest.
(Also useful for bagging trig points if you're into that sort of thing.)
Great app, wish I'd downloaded it before now.
I don't think there's an iOS version unfortunately.